An apparatus consumes energy during operation and produces heat. To maintain the apparatus in a stable working state, it is necessary to keep its temperature within a desired range. Therefore, there is a need to detect ambient temperature variation surrounding the apparatus and timely control the temperature of the apparatus. FIG. 1 is a first conventional temperature control circuit for a general cooling fan, and mainly includes a resistor 111 serially connected to a thermal resistor 112. When a constant voltage Vcc is applied across the circuit, the thermal resistor 112 varies its resistance with variation of temperature, and a voltage Vo at an output of the circuit varies with changes in the resistance of the thermal resistor 112. Wherein, the voltage Vo at the output varies at a fixed gradient.
FIG. 2 is a second conventional temperature control circuit for a general cooling fan, and mainly includes two parallelly connected thermal resistors 112′ and a resistor 111 serially connected to the two thermal resistor 112′, so that a voltage Vo at an output of the parallelly connected thermal resistors 112′ and the resistor 111 after a voltage division varies with the resistance of the parallelly connected thermal resistors 112′. However, the resistance of the parallelly connected thermal resistors 112′ at room temperature is generally the same as that of the circuit with one single thermal resistor 112 as shown in FIG. 1.
FIG. 3 is a third conventional temperature control circuit for a general cooling fan, and mainly includes two serially connected thermal resistors 112′ and a resistor 111 serially connected to the two thermal resistor 112′, so that a voltage Vo at an output of the serially connected thermal resistors 112′ and the resistor 111 after a voltage division varies with the resistance of the serially connected thermal resistors 112′. However, the resistance of the serially connected thermal resistors 112′ at room temperature is generally the same as that of the circuit with one single thermal resistor 112 as shown in FIG. 1.
Please refer to FIG. 4 along with FIGS. 1, 2, and 3. When the temperature changes from T1 to T2, both the changes of the voltage Vo at the output of several parallelly or serially connected thermal resistors 112′ and one serially connected resistor 111 obtained from voltage division, as shown by the oblique lines b and c in FIG. 4, and the changes of the voltage Vo at the output of serially connected one thermal resistor 112 and one resistor 111, as shown by the oblique line a in FIG. 4, have a fixed gradient. That is, the conventional temperature control circuits do not change the gradient of output to temperature variation to enable changes of sensitivity of the circuit to the temperature variation. For example, in actual applications, a fan driven by the output voltage Vo of any of the above-described temperature control circuits does not automatically switch to different rotary speeds within a fixed range of temperature variation, and therefore could not satisfy a user's need. It is therefore desirable to develop a circuit structure to eliminate the drawbacks existed in the conventional temperature control circuits.